The present invention relates to an implantable electrical device, e.g., an implantable medical device such as an implantable cochlear stimulation system, which receives its operating power and/or which receives recharging power from an external (non-implanted) power source.
Implantable electrical devices are used for many purposes. A common type of implantable device is a tissue stimulator. A tissue stimulator includes one or more electrodes in contact with desired tissue. An electrical stimulation current is generated by the stimulator and applied to the tissue through the electrode(s).
In order for an implanted device to perform its intended function, e.g., to generate an electrical stimulation current, it needs a power source. Some implanted devices, e.g., cardiac pacemakers, employ a high capacity battery that has sufficient power stored therein to provide operating power for the device for several years. Other implanted devices, e.g., a cochlear stimulation system, do not use an implanted power source bur rather receive a continuous stream of power from an external source through an rf or inductive link. Yet other implanted devices include a rechargeable power source, e.g., a rechargeable battery, that must be regularly recharged, e..g, once a day, or 2-3 times per week, from an external source in order for the implanted device to operate. The present invention is intended for use with the latter two types of implanted devices, e.g., those that receive a continuous stream of operating power from an external source, or those that must receive power at regular intervals in order to recharge an implantable power source.
Power is typically coupled to an implanted device through inductive coupling. That is, an external coil receives an ac power signal. An implanted coil connected to, or forming part of, the implantable device, is placed in close proximity to the external coil so that magnetic flux generated by the ac power signal in the external coil induces an ac power signal in the second coil, much like the primary winding of a transformer couples energy to a secondary winding of the transformer, even though the two windings are not electrically connected to each other. When coupling power to an implanted device in this manner, an optimum power transfer condition exists only when there is a good impedance match between the implant device and the external device. While impedance matching schemes can and have been used in the external device, such matching schemes are only effective for a given distance between the external coil and the implant coil, and for a given load attached to the implant device.
Unfortunately, neither the load associated with the implant device nor the separation distance between the external coil and the implant coil are constants. Each of these parameters are, in practice, variables, that may vary, e.g., from 3-to-15 mm for the separation distance, and 20 to 300 ohms for the load. As a result, optimum power transfer between the external device and implant device is rarely achieved. Thus, a less than optimum power transfer condition exists and much of the energy sent to the external coil is lost. What is needed, therefore, is a way to assure that optimum power transfer conditions exist between the external coil and implant device at the time a power transfer is made.
For many implant devices, optimum power transfer has heretofore generally not been a serious concern inasmuch as the external device (which has generally comprised a relatively large device that is worn or carried by the patient) has been viewed as having a potentially infinite power source (through recharging and/or replacing its battery). Unfortunately, however, transferring large amounts of power without concern for how much power is lost is not only inefficient, but may create regulatory problems. That is, most regulatory agencies stipulate the power levels that may be used with an implant device.
Further, new generation external devices are being made smaller and smaller to accommodate the needs and desires of the user. For example, a behind-the-ear (BTE) external device may be used with an implantable cochlear stimulator (ICS). Such BTE external device that is about the same size as a conventional behind-the-ear hearing aid. Such smaller devices, as a practical manner, do not have a potentially infinite power source, but must be powered using a small button battery, or equivalent. Such small battery must provide power for both the external unit and the implant unit, and achieving an efficient power transfer is a key element in assuring a long battery life.
It is known in the art, see, e.g., U.S. Pat. No. 4,654,880, to include the external coil and implant coil (as coupled to each other based on a given separation distance and load) in the oscillator circuit that sets the frequency of the signal that is coupled between the external coil and implant coil. Such circuit is somewhat self-compensating because as the transfer efficiency starts to go down (e.g., because the separation distance changes, or because the load changes) the frequency of the signal used to couple energy into the implant coil automatically changes in a direction that tends to retune the coupled coils so that the energy transfer becomes more efficient.
Disadvantageously, changing the frequency of the signal coupled into the implant circuit may also create regulatory problems. That is, regulatory agencies may be very strict about the frequencies of signals that are allowed to be transmitted, even if only transmitted over short distances.
In view of the above, it is evident that what is needed is a transmission scheme for use with a medical implant device that optimally transfers power to the implant device from an external device at a fixed frequency, i.e., that transfers power into the implant device from the external device with minimum power loss. The present invention addresses this and other needs.